The regions in the disk responsible for flickering are hot
[1]}, [2]}, and their emission is
consequently rising toward the blue; that of the giant rises instead toward
the red. Therefore, to reduce the background glare of the giant it is
convenient to go as blue as possible [3]}. Detection
of flickering at satellite ultraviolet wavelengths is straightforward as
demonstrated by the Swift observations of SU Lyn by Mk16. On
ground-based observations, the bluest photometric band is \(U\)  (in its many
variants: Landolt's \(U\) , \(u\)  from Stromgren, or SLOAN \(u^{\prime }\) ). \(U\)  band was
relatively easy to observe in the old times of photo-electric photometers
mounted on all-reflecting telescopes with bare-aluminum surfaces, thanks to
the high instrumental throughput (no coatings or refractive components) and
the excellent sensitivity of photo-multipliers like the standard RCA 31034A
and Hamamatsu R934-02. Many searches for flickering in symbiotic stars were
performed at that time (e.g. [4]}, [5]}, [6]}. The advent of CCDs, with their
much lower sensitivity at \(U\) -band wavelengths, badly affected the study of
flickering, with rarer attempts carried out (e.g.
[7]}, [8]}, [9]}, [10]}). To further complicate the matter, the sensitivity of
CCDs, that extends up and beyond 1 \(\mu \) m, allows signal to be recorded from
the red-leak affecting most \(U\)  filters, especially those of the modern
multi-layer dielectric type. Such a red-leak may be so severe in the case
of cool giants, that the fraction of recorded photons coming through the
red-leak easily outnumbers those going through the proper transmission
profile of the \(U\) -band [11]}.
